148 PROTECTION AGAINST CHILLING [CH. XIV 



they are specially adapted to avoid excessive illumination. 

 Support for this explanation is found in the similar edge- 

 on position of many ordinary leaves of alpine and prairie 

 plants, &c, developed in intensely lighted regions : thus 

 the leaves of species of Silphium (so-called " Compass 

 plants "), Lactuca, &c, are twisted on the petiole and 

 placed in the plane of the meridian, and the isobilateral, 

 erect leaves of Iris, Narthecium, Tofieldia, Phormium, &c, 

 also tend to put themselves edge-on to the light. 



It should be noticed, however, that the assumption of 

 the vertical position by ordinary leaves, cotyledons, &c, is 

 often brought about temporarily by movements of the 

 petiole and pulvinus, as already explained. Thus the 

 cotyledons of many Crucifers, Leguminosse, Composite, 

 &c, are horizontal during the day and vertical at night. 

 Obviously there is here no question of avoiding excessive 

 illumination, and Darwin has shown that chilling by ra- 

 diation is the danger escaped by this adaptation. Similarly 

 in the case of leaves which either erect their lamina, e.g. 

 species of Coronilla, Mimosa, Acacia, Gleditschia, Nicotiana, 

 Marsilea, &c, or let it hang pendent during the night, 

 e.g. Clover, Lotus, Oxalis, Averrhoa, Lupinus, Melilotus, 

 Desmodium, the vertical position of the leaf-surface lessens 

 the danger of chill. 



Nevertheless these cases do not contradict those where 

 the vertical position is assumed by leaves during the in- 

 tense heat or glare of the tropical day, such as those re- 

 ferred to, and Leucadendron, Melaleuca, Protea, Grevillea, 

 Banksia, &c. 



